The density and temperature structure near the exobase of Saturn from Cassini UVIS solar occultations

Abstract

We analyzed 15 solar occultations observed by the Cassini UVIS instrument to constrain the density and temperature structure near the exobase of Saturn. We retrieved the density of H2 and thus the temperature at altitudes higher than 1900km above the 1bar level by analyzing the ionization continuum of H2 at wavelengths shorter than 804Å. We find that the exospheric temperature ranges from 370K to 540K, with a typical uncertainty of less than 20K. According to our data the temperature increases with latitude from the equator to the poles by 100–150K. At similar latitudes, the temperature varies by 20–50K at different times with no evidence for any systematic diurnal trend so far. Based on our data, the exobase of Saturn is 2700–3000km above the 1bar level and the thermal escape parameter near the exobase ranges from 260 to 340, implying that thermal escape from Saturn is firmly in the Jeans regime. The mixing ratio of H2 is close to unity at all altitudes below the exobase. We find that the pressure levels in the thermosphere deviate significantly from a simple spheroid predicted by potential theory. This is consistent with significant meridional temperature variations in the lower thermosphere. A global analysis of the temperature structure at different depths in the atmosphere is required to constrain both the shape and the deposition and redistribution of energy in the upper atmosphere further.